oh oh oh! okay so, I've been drawing most of my life and i play a lot of games too, so I've even had my share of tendonitis n such.
step one to any pain is sadly to stop completely. avoid what's causing the issue till its healed.
then stretches, there's many ways to stretch your hands and wrists, do them often, at-least hourly while working. and be gentle! work your way up
and finally what causes many issues (the ones that aren't genetic like some forms of arthritis) is simply the overworking of the tendons and muscles that pull said tendons in the arms and hands. just constant repetition is not good. so I've gotten not only grip strength devices to help work out these specific areas, but I started bouldering years ago and love it! I go to the gym 3 times a week and climb walls and its made it so easy to just draw aaaall day every day for me ;D